void main(String[] args) { letsHear(new Cat()); letsHear(new Dog());}

In another example, if Number, Rational, and Integer are types such that ''Number'' :> ''Rational'' and ''Number'' :> ''Integer'' (Rational and Integer as subtypes of a type Number that is a supertype of them), a function written to take a Number will work equally well when passed an Integer or Rational as when passed a Number. The actual type of the object can be hidden from clients into a black box, and accessed via object